1. Technical Field of the Invention
The present invention relates to a cellular telecommunications network and, in particular, to a cell pattern within such a network using an adaptive frequency reuse plan.
2. Description of Related Art
Frequency reuse patterns are cell-based schemes for assigning the frequency channels available within a particular cellular telecommunications system. The most basic unit of any frequency reuse pattern is a cell. Each cell within a frequency reuse pattern is assigned a number of frequency channels. A plurality of cells are then associated together and referred to as a cluster and utilizes all of the frequency channels available to a particular cellular telecommunications system. Groups of clusters are then used to provide a cellular coverage area within the cellular telecommunications system and the frequency channels allocated for one cluster are reused in other clusters. The scheme for recycling or reassigning the frequency channels throughout the serving coverage area is referred to as a reuse plan. The distance between a first cell using a particular frequency channel within a first cluster and a second cell using the same frequency channel within a second cluster is further known as a reuse distance.
The reuse of the same frequency channels by a number of different cells implies that cells may suffer from co-channel interferences. It is therefore desirable for the received strength of the serving carrier (C) within each cell to be higher than the total co-channel interference level (I). As a result, the higher the carrier to interference (C/I) value, the better the speech quality. A higher C/I value is obtained partly by controlling the channel reuse distance. The larger the reuse distance between adjacent cells utilizing the same frequency channels, the lesser the co-channel interferences created between those cells.
The C/I ratio is further related to a frequency reuse plan (N/F) where N indicates the number of cells included within a single cluster and F indicates the number of frequency groups. For example, the C/I ratio is directly related to the following equation: EQU D.sub.R =(3*F).sup.1/2 *R
Where:
D.sub.R is the reuse distance; PA1 F is the number of frequency groups; PA1 R is the radius of a cell.
Accordingly, the larger the F value, the greater the reuse distance. However, it is not always desirable to use a larger F value to increase the C/I ratio. Since the total number of available frequency channels (T) is fixed within a particular mobile network, if there are F groups, then each group will contain T/F channels. As a result, a higher number of frequency group (F) would result in a fewer channels per cell and lesser call capacity.
For most cellular systems, capacity is not a major issue when the system initially goes into operation. Therefore, in order to achieve a high C/I value and to improve the quality of speech connection, a high frequency reuse plan (N/F), such as 9/27, is initially used. However, as the capacity increases, the cellular telecommunications network has to resort to a lower frequency reuse plan, such as a 7/21 or 4/12, to allocate more frequency channels per cell. Consequently, the whole cellular telecommunications network and its associated clusters and cells need to be reconfigured with a new frequency reuse plan. Such reconfiguration and reallocation requires an investment of considerable time and resource. On the other hand, due to poorer speech connection quality, it is undesirable to use a low frequency reuse plan from the beginning when there is no need for high capacity.
Accordingly, there is a need for a mechanism to enable service operators to adapt their frequency plan according to their capacity and C/I without re-configuring the channel allocation.